Hematologic malignancies are forms of cancer that begin in the cells of blood-forming tissue, such as the bone marrow, or in the cells of the immune system. Examples of hematologic cancer are acute and chronic leukemias, lymphomas, multiple myeloma and myelodysplastic syndromes.
Myeloproliferative neoplasms, or MPNs, are hematologic neoplasms that arise from neoplastic hematopoietic myeloid progenitor cells in the bone marrow, such as the precursor cells of red cells, platelets and granulocytes. Proliferation of neoplastic progenitor cells leads to an overproduction of any combination of white cells, red cells and/or platelets, depending on the disease. These overproduced cells may also be abnormal, leading to additional clinical complications. There are various types of chronic myeloproliferative disorders. Included in the MPN disease spectrum are Essential Thrombocythemia (ET), Polycythemia vera (PV), ChronicMyelogenous Leukemia (CML), myelofibrosis (MF), chronic neutrophilic leukemia, chronic eosinophilic leukemia and acute myelogenous leukemia (AML). A myelodysplastic syndrome (MDS) is a group of symptoms that includes cancer of the blood and bone marrow. Myelodysplastic syndromes (MDS) includes diseases such as, refractory anemia, refractory anemia with excess blasts, refractory cytopenia with multilineage dysplasia, refractory cytopenia with unilineage dysplasia, and chronic myelomonocytic leukemia (CMML).
Essential Thrombocythemia
Circulating blood platelets are anucleate, although they retain small amounts of megakaryocyte-derived mRNAs and a fully functional protein biosynthetic capacity (Gnatenko et al., Blood 101, 2285-2293 (2003)). Essential Thrombocythemia (ET) is a myeloproliferative disorder subtype, characterized by increased neoplastic proliferation of megakaryocytes, elevated numbers of circulating platelets, and considerable thrombohemorrhagic events, not infrequently neurological (Nimer, Blood 93, 415-416 (1999)). ET is seen with equal frequency in males and females, although an additional female incidence peak at age 30 may explain the apparent higher disease prevalence in females after this age. The molecular basis of ET remains to be established, although historically it has been considered a “clonal” disorder (El-Kassar et al., Blood 89, 128 (1997); “Evidence that ET is a clonal disorder with origin in a multipotent stem cell” P J Fialkow, Blood 1981 58: 916-919). Other than the exaggerated platelet volume evident in subsets of ET platelets, the cells remain morphologically indistinguishable from their normal counterparts. No functional or diagnostic test is currently available for ET, and it remains to be diagnosed by exclusion of other potential hematological disorders Incidence estimates of 2-3 cases per 100,000 per year are consistent with other types of leukemia, but prevalence rates are at least ten times higher due to the low mortality rates associated with ET.
Current therapies for ET focus primarily on prevention of thrombotic/hemorrhagic occurrence and involve non-specific reduction of blood platelet levels. However, none of these existing therapies focus specifically on the neoplastic progenitor cells driving the malignancy responsible for the disease state. For example, treatment of ET with cytotoxic chemotherapy debulks neoplastic cells while leaving residual progenitor cells in place. This results in new neoplastic cells arising from the progenitor cells and continuation of the disease state. Additionally, many individuals with ET develop resistance to front-line treatments such as hydroxyurea or discontinue use of these drugs altogether due to adverse side effects.
Polycythemia Vera
Patients with Polycythemia Vera (PV) have marked increases of red blood cell production. Treatment is directed at reducing the excessive numbers of red blood cells. PV can develop a phase late in their course that resembles primary myelofibrosis with cytopenias and marrow hypoplasia and fibrosis. The Janus Kinase 2 gene (JAK2) gene mutation on chromosome 9 which causes increased proliferation and survival of hematopoietic precursors in vitro has been identified in most patients with PV. Patients with PV have an increased risk of cardiovascular and thrombotic events and transformation to acute myelogenous leukemia or primary myelofibrosis. The treatment for PV includes intermittent chronic phlebotomy to maintain the hematocrit below 45% in men and 40% in women. Other possible treatments includes hydroxyurea, interferon-alpha, and low-dose aspirin.
Myelofibrosis
Myelofibrosis or MF, or primary myelofibrosis is a myeloproliferative neoplasm in the same spectrum of diseases as ET. Patients with MF often carry the JAK2 V617F mutation in their bone marrow. Occasionally ET evolves into MF. JAK2 inhibition is currently considered a standard of care for MF in countries where ruxolitinib (Jakafi®), a janus kinase inhibitor, is approved. There is no evidence that JAK2 inhibitors, such as Jakafi®, selectively inhibit proliferation of the leukemic clone responsible for the disease and thus, they may not be “disease modifying”.
Acute Myelogenous Leukemia
Acute Myelogenous Leukemia (AML) is a cancer of the myeloid line of blood cells. AML is the most common acute leukemia affecting adults. Patients with AML have a rapid growth of abnormal white blood cells that accumulate in the bone marrow and interfere with the production of normal blood cells. Replacement of normal bone marrow with leukemic cells causes a drop in red blood cells, platelets, and normal white blood cells. The symptoms of AML include fatigue, shortness of breath, easy bruising and bleeding, and increased risk of infection. As an acute leukemia, AML progresses rapidly and is typically fatal within weeks or months if left untreated. The standard of care for AML is treatment with chemotherapy aimed at inducing a remission; patients may go on to receive a hematopoietic stem cell transplant.
Myelodysplastic Syndrome
A myelodysplastic syndrome (MDS) is a group of symptoms that includes cancer of the blood and bone marrow. Myelodysplastic syndromes (MDS) includes diseases such as, refractory anemia, refractory anemia with excess blasts, refractory cytopenia with multilineage dysplasia, refractory cytopenia with unilineage dysplasia, and chronic myelomonocytic leukemia. The immature blood stem cells (blasts) do not become healthy red blood cells, white blood cells or platelets. The blast die in the bone marrow or soon after they travel to the blood. This leaves less room for healthy white cells, red cells and/or platelets to form in the bone marrow.
The myelodysplastic syndromes (MDS) are a collection of hematological medical conditions that involve ineffective production of the myeloid class of blood cells. Patients with MDS often develop severe anemia and require frequent blood transfusions. Bleeding and risk of infections also occur due to low or dysfunctional platelets and neutrophils, respectively. In some cases the disease worsens and the patient develops cytopenias (low blood counts) caused by progressive bone marrow failure. In some cases the disease transforms into acute myelogenous leukemia (AML). If the overall percentage of bone marrow myeloblasts rises over a particular cutoff (20% for WHO and 30% for FAB), then transformation to acute myelogenous leukemia (AML) is said to have occurred.
What is needed, therefore, are new treatments for myelodysplastic proliferative disorders or neoplasm such as ET, PV, MF, CML and AML, and for myelodysplastic syndrome which target the neoplastic progenitor cells responsible for the disease's malignant phenotype, particularly in individuals who are resistant to or experience adverse events as a result of taking commonly prescribed front-line therapies for this disorder.
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